The progress of lasers in recent years has been remarkable and a high output and compact solid laser or semiconductor laser having a light emission region in the range of from near infrared to infrared has become easily available. These lasers are very useful as a light exposure source in the case of directly making a printing plate from digital data of a computer or the like.
The positive working printing plate material for an infrared laser contains an alkali aqueous solution-soluble binder resin, an IR dye generating heat by absorbing light, etc., as the indispensable components, in the unexposed areas (image areas), the IR dye, etc., functions as a dissolution inhibitor of substantially lowering the solubility of the binder resin by the interaction with the binder resin, and in the exposed areas (non-image areas), the interaction of the IR dye, etc., and the binder resin is weakened by the action of the generated heat, whereby the exposed areas are dissolved in an alkali developer to form a lithographic printing plate.
However, in such a lithographic printing plate precursor for infrared laser, there is a problem that the difference between the dissolution resistance of the non-exposed areas (image areas) to the developer and the solubility of the exposed areas (non-image areas) in various using conditions is yet insufficient, and an excessive development or an inferior development is liable to occur by the fluctuation of the using conditions. Also, since the image-forming faculty of a lithographic printing plate precursor depends upon the heat generation at the surface of a recording layer by an infrared laser exposure, there is also a problem that in the vicinity of a support, the quantity of heat used for the image formation, that is, used for dissolving the recording layer is reduced by the diffusion of heat to reduce the difference between the exposed area and the unexposed area, whereby the reproducibility of highlight is insufficient.
For example, about the development latitude, in a positive working lithographic printing plate precursor material of making a printing plate by a UV exposure, that is, a conventional positive working lithographic printing plate precursor material containing an alkali aqueous solution-soluble binder resin and an onium salt or quinonediazide and having a function that the onium salt or quinonediazide functions as a dissolution inhibitor by the interaction with the binder resin in the non-exposed areas (image areas), and is decomposed by the action of light to generate an acid and functions as a dissolution accelerator in the exposed areas (non-image areas), there scarcely occur problems. However, since in the positive working lithographic printing plate precursor material, the UV exposure is imagewise carried out via a lith film and the diffraction of light is liable to occur, there is a problem in the highlight reproducibility.
On the other hand, in a positive working lithographic printing plate precursor material for an infrared laser, an infrared absorbent, etc., only functions as a dissolution inhibitor of the non-exposed areas (image areas) and does not accelerate to the dissolution of the exposed areas (non-image areas). Accordingly, for obtaining the difference in solubility between the non-exposed areas and the exposed areas in the positive working lithographic printing plate precursor material for infrared laser, the binder resin previously having a high solubility in an alkali developer cannot but use, whereby there are problems that the lithographic printing plate precursor material becomes inferior in the scratch resistance and unstable in the state before development.
Furthermore, in a double-layer type photosensitive layers (heat-sensitive layers) as the present invention, even when ununiformities of the surface state slightly generate on the lower layer and upper layer in a level of causing no problem, they are amplified by the existence of many layers, and although it is difficult to find them as surface fault, after development, they are actualized as the ununiformity of the image reproducibility or the unevenness of dot image area. Therefore, with regard to the plane ununiformities of the lower layer and upper layer, the higher level than usual is required.